A typical system to which this invention applies is one involving a wet well that more or less continuously receives liquid (such as sewage) at an inlet location, and having two or more separate pumps each drawing liquid from the wet well, and each pumping into a common manifold or conduit. It is well known that, if the backpressure into which a pump is pumping is too high, the pump will lose its pumping capability. Similarly, where one or more pumps are already pumping into a common conduit, from this pump station or any other pump station in the system (conduit), causing a backpressure at a certain level, it is possible that the start of a further pump from this pump station or any other pump station in the system will so increase the backpressure that there will be no effective increase in the total pumping capacity.
A further disadvantage of conventional systems utilizing two or more pumps lies in the fact that, depending upon how the pump selection is configured, one of the pumps may be operated for a much greater percentage of the total time than another of the pumps.
A still further disadvantage of conventional systems arises when there is an electric power failure. When the electric power is restored the logic may call for one or more or all of the pumps to start simultaneously, and this will create a sudden pressure surge in the form of backpressure. Part of the solution to the first problem set out above is to program a memory with high and low values for each pump, which will cancel a called-for pumpstart if the backpressure is too high, or will turn off a pump if the backpressure rises beyond the "high" value for that pump stored in memory. In the event of a sudden backpressure surge upon restoration of electrical energy after energy failure, the surge could trigger the pumps to be turned off, and this would initiate an on-off cycle which would be very damaging to the system.
A further disadvantage of conventional systems is the lack of an accurate measurement of inflowing liquid, particularly when the system is operating at or near its top capacity.
A final disadvantage of present sewage pumping systems deserves mention. Since pumps start at random, it can happen that all pump stations on the same force main are called to pump simultaneously. They then pump into a high backpressure and the treatment plant receives a large volume of sewage which interferes with the treatment process and will result in an unnecessarily high electricity consumption because of the high backpressure. Due to these disadvantages, many engineers specify the use of the very expensive variable speed pump.